The present invention is directed to windback devices, employed with a rotating shaft and positioned between a lubrication sump and a shaft seal for the sump, and comprises an improved windback for preventing a lubrication fluid from reaching the seal.
With rotating shafts, lubrication sumps are frequently required along segments thereof to provide lubrication for shaft bearings and the like. It is frequently desirable to isolate the lubrication sump from the remaining media in which the shaft is immersed as well as to ensure that the lubricant remains in the sump so that (1) the lubrication of the bearings may continue and (2) the lubricant does not contaminate the other media in which the shaft is immersed. For example, in gas turbine applications, the lubrication sump is isolated from the remainder of the gas turbine housing by the shaft seal and the remainder of the housing is filled with high temperature, high pressure gases at temperatures and pressures higher than those desired in the lubrication sump. It is important to prevent lubricant from mixing with the high temperature gases which when an oil lubricant is used, could form oil coke, a byproduct of oil heated to an elevated temperature which chemically alters the oil and is detrimental to the gas turbine. Oil coke could foul seal surfaces and prevent proper bearing lubrication.
To accomplish such isolation, a shaft seal is normally provided adjacent the sump to retain the lubricant therewithin. However, for rotating shafts, the shaft windage tends to move the lubricant along the shaft out of the sump (termed "lubricant splash") which could result in the loss of lubricant. Sealing means including a shaft seal are normally provided to prevent lubricant loss. The shaft seal, which may comprise a segmented carbon rubbing seal, may have the carbon sealing faces contaminated by the lubricant should the same reach the seal. In order to prevent the lubricant splash from reaching the seal carbons or the like, a windback device is interposed along the shaft between the seal and the lubrication sump. The windback device normally comprises an annular collar-like member receiving the shaft in the opening thereof and having a screw pitch or thread in the surface thereof facing the shaft. The windback device is secured to the housing to prevent leakage from the sump to the remainder of the housing except along the shaft, and the shaft seal is interposed between the windback device and the shaft on the side of the windback device remote from the sump to prevent lubricant leakage along the shaft.
Since the windback device is in close proximity to the shaft yet not so close as to expect to create a pressure drop therealong, it would be expected that the pressure along the opening in the windback device, i.e., at positions adjacent the rotating shaft, would be essentially the same so that any oil splash moving along the shaft of the windback device would be encouraged to flow back toward the sump along the threaded inner surface of the windback collar. A number of pressure measurements along the axial length of the windback device were performed with the surprising result that a pressure drop actually existed between the sump and the end of the windback device remote from the sump. The pressure at the sump end of the windback opening was greater than the pressure at the seal end of the windback opening. Thus, lubricant would be encouraged by this pressure drop to move in the direction away from the sump along the shaft to other interior portions of the shaft housing because of the pressure drop along the windback in that direction. In applications such as gas turbines, the pressure of the housing medium in regions other than in the sump is higher than the pressure within the sump. Thus, one would expect that the pressure in the sump adjacent the seal would be higher than the pressure in the remainder of the sump. These experiments showed that, even in such applications, the pressure in the sump adjacent the seal was lower than the pressure in the remainder of the sump, thus incurring additional lubricant flow toward the seal.
By virtue of this discovery, this invention proposes to modify the windback device to eliminate any such pressure drop along it, and thereby reducing the possibility of lubricant leakage along the shaft out of the sump through the seal, thus ensuring the return to the sump of lubricant caused by shaft rotational splash.